1. Technical Field
The present invention relates to a fuel cell comprising a polymer electrolyte for use in portable power sources, power sources for electric vehicles, cogeneration systems for home and the like and a method for producing the same. More specifically, the present invention relates to an electrode for a fuel cell.
2. Background Art
A fuel cell using a polymer electrolyte is an electrochemical device for generating electric power and heat at the same time by electrochemically reacting a fuel gas containing hydrogen with an oxidant gas containing oxygen such as air.
In constructing such a fuel cell, first, catalytic reaction layers mainly composed of a carbon powder with a platinum group metallic catalyst carried thereon are formed on both sides of a polymer electrolyte membrane which transfers selectively a hydrogen ion. Next, on the outer surfaces of the catalyst layers, a pair of gas diffusion layers having both fuel gas permeability and electronic conductivity are formed, and each electrode is formed in combining one of the gas diffusion layers with one of the catalytic reaction layers. Thus, the electrodes and the polymer electrolyte membrane are primarily configured in one unit. This is called membrane electrode assembly (hereinafter, this is also referred to as xe2x80x9cMEAxe2x80x9d). In order to avoid leaking outside of supplied fuel gas or oxidant gas, or mixing of the two kinds of gases together, a gas seal material or a gasket is placed in the circumference of the electrodes so that they sandwich the polymer electrolyte membrane. Then, a large number of MEA""s are laminated with electroconductive separator plates interposed between each thereof, and thereby a fuel cell as so-called laminated cell is condigured.
Next, the catalytic reaction layers in the electrodes of the fuel cell will be described. The carbon powder with metallic catalyst carried thereon is in the form of particle falling in the range of several hundred angstroms to several microns. By using a mixture of this carbon powder and a dispersion of a polymer electrolyte, a catalytic reaction layer having a thickness of 30 to 100 microns is formed between the electrode and the solid electrolyte membrane by a coating process such as printing or the like. In this catalytic reaction layers, electrochemical reaction of the fuel gas and the oxidant gas proceeds.
For example, in the anode where hydrogen reacts, hydrogen gas is supplied to the electrode surface through a fuel gas supply path notched in the separator plate. The electrode is usually made of a gas-permeable electroconductive material such as carbon paper or carbon cloth, and hydrogen gas can reach the catalytic reaction layer by permeating the electrode. Onto the surface of the catalyst carrying carbon powder, a polymer electrolyte formed with a dried and solidified solution of the polymer electrolyte adheres. In a so-called three phase zone constituted by a vapor phase containing hydrogen gas, a solid phase of the catalyst carrying carbon powder, and a phase of the polymer electrolyte, all of which being close to each other, the hydrogen gas is oxidized to become hydrogen ions and is discharged into the polymer electrolyte. The electron generated by oxidation of the hydrogen gas moves to an outside electric circuit passing through the electroconductive carbon powder. This electrochemical reaction progresses in a broader area because of the hydrogen gas dissolved in the polymer electrolyte. The thickness of the catalytic reaction layer is varied according the production process thereof; however, in order to obtain a good cell performance, the catalytic reaction layer is usually designed to have a thickness of 30 to 100 microns.
Within the catalytic reaction layer, however, the area which contributes to the actual electrode reaction is considered only a part of 20 microns thick in contact with the polymer electrolyte membrane. This is because the generated hydrogen ion has a difficulty in reaching the polymer electrolyte membrane. Also, in the condition where the catalyst carrying carbon powder is not in electrical contact with other carbon powder or with the electroconductive electrode, although the hydrogen ion can easily move, the electron is prevented from moving to the outside circuit. As a result, there has been a problem that the catalytic reaction layer formed by coating comes to the state where a large part thereof does not contribute to the electrode reaction, which impairs its performance, and therefore a large quantity of platinum is needed for recovering its performance.
As a consequence, it is desired to improve the catalytic reaction layer and make the platinum catalyst effectively contribute to the electrode reaction, thereby to improve the utilization rate of the platinum catalyst.
The electrode for use in the polymer electrolyte fuel cell is produced by forming catalytic reaction layer comprising a noble metal carrying carbon powder on electroconductive porous electrode supporting material serving as gas diffusion layer. As the porous electroconductive supporting material, a carbon paper, a carbon cloth or the like made of carbon fiber or the like is used. These electrodes are generally formed on the supporting materials by means of screen printing process or transcription process with a noble metal carrying carbon fine powder prepared into an ink using an organic solvent such as isopropyl alcohol.
In recent years, in the viewpoint of workability, inks for electrodes using aqueous solvents in place of organic solvents have been proposed. However, when these methods are used, a part of the noble metal carrying carbon powder serving as the catalyst in the electrode penetrates into the electrode supporting material, which constitutes the gas diffusion layer. For this reason, measures are required such as using a relatively large amount of electrode catalyst, or maintaining electroconductivity in the contact surface between the gas diffusion layer and the catalytic reaction layer by increasing cramping pressure of the cell. Alternatively, a method in which the electrode catalyst layers are primarily applied and formed on the polymer electrolyte membrane has been proposed. These electrodes and the polymer electrolyte membrane are bound to each other by means of hot-pressing or the like.
As described above, in the polymer electrolyte fuel cell, it is strongly required not only to increase the utilization rate of the catalyst in the catalytic reaction layer but also to decrease the contact resistance between the carbon paper or the carbon cloth, which constitutes the gas diffusion layers, and the catalytic reaction layers.
In the electrode as constituent of a polymer electrolyte fuel cell, the surface area of a so-called three phase zone, which is formed by a micropore serving as a supply path for the reaction gases (fuel gas and oxidant gas), the hydrogen ion conductive polymer electrolyte, and the electronic conductive electrode supporting material, is an important factor, which influences the discharge characteristic of the cell.
Conventionally, with an aim to enlarge this three phase zone, attempts have been made to provide, on the interface between the polymer electrolyte membrane and the porous electrode supporting material, a layer comprising a material which constitute the electrode supporting material and the polymer electrolyte, mixed with each other and dispersed. For example, in the technique described in Japanese Patent Publications No. Sho 62-61118 and Japanese Patent Publication No. Sho 62-61119, a method is disclosed in which a mixture of a polymer electrolyte dispersion and a catalyst compound is applied on the polymer electrolyte membrane, and this is hot-pressed with the electrode supporting material and subsequently the catalyst compound is reduced. Also suggested is a method in which, after a catalyst compound is reduced, this is applied onto an electrolyte membrane and hot-pressed.
Further disclosed in Japanese Patent Publication No. Hei 2-48632 is a method in which, after porous electrode supporting material is molded, a solution containing a dispersed resin, which constitutes an ion exchange membrane, is sprinkled onto the electrode supporting material and these electrode and the ion exchange membrane are hot-pressed. Still further, a powder comprising a polymer resin powder with a polymer electrolyte applied on the surface thereof is proposed in Japanese Laid-Open Patent Publication No. Hei 3-184266 and a method of mixing a powder of a polymer electrolyte into the electrode is proposed in Japanese Laid-Open Patent Publication No. Hei 3-295172, respectively. Moreover, a method of mixing a polymer electrolyte, a catalyst, a carbon powder, and a fluorocarbon resin and forming them into a membrane for use as the electrode is disclosed in Japanese Laid-Open Patent Publication No. Hei 5-36148. In the above techniques, alcohol is used as a solution for forming a polymer electrolyte in the electrode.
Also disclosed in U.S. Pat. No. 5,211,984 is a method of preparing a dispersion in the form of an ink by dispersing a polymer electrolyte, a catalyst and a carbon powder in glycerin or tetrabutyl ammonium salt as a solvent, applying and forming the dispersion on a film made of polytetrafluoroethylene (hereinafter, this is referred to as xe2x80x9cPTFExe2x80x9d) and subsequently transcribing this film onto the polymer electrolyte membrane surface. Further reported is a method of replacing an exchange group of the polymer electrolyte membrane with a substituent group of Na type, applying the ink-like dispersion as described above onto the surface of this membrane, heating and drying the same at a temperature of 125xc2x0 C. or higher, and replacing again the exchange group with H type.
On the other hand, in order to realize a high output density, which is a feature of the polymer electrolyte fuel cell, it is important to form a supply path (gas channel) for the reaction gas in the catalytic reaction layer and improve permeability and diffusion abilities of the gas. Thus, attempts have been made to add a water repellent material such as a fluorocarbon resin to the catalyst reaction layer thereby to form a gas channel. For example, in Japanese Laid-Open Patent Publication No. Hei 5-36418, proposed is a method of forming a catalyst layer by dispersing PTFE powder and a catalyst carrying carbon powder in a solution of a polymer electrolyte and kneading the dispersion. Also proposed in Japanese Laid-Open Patent Publication No. Hei 4-264367 is forming an electrode by using a mixture of a catalyst carrying carbon powder and a colloidal dispersion of PTFE.
Further, in J. Electroanal. Chem. No. 197 (1986), page 195, a method is proposed for forming a gas diffusion electrode for acid electrolyte solution by mixing a carbon powder subjected to water repellency treatment with PTFE with a catalyst carrying carbon powder. On the other hand, in U.S. Pat. No. 5,211,984, a method is proposed for forming a catalyst layer in the electrode only with a polymer electrolyte, a catalyst and a carbon powder without using a water repellent material as described above.
However, when a catalyst carrying carbon powder and a water repellent material such as a fluorocarbon resin or the like, or a carbon powder subjected to water repellency treatment are added at the same time into a dispersion of a polymer electrolyte, a large quantity of polymer electrolyte is adsorbed onto the water repellent material or the carbon powder subjected to water repellency treatment. At this time, larger the amount of the polymer electrolyte adsorbed onto the carbon powder, the degree of contact between the polymer electrolyte and the catalyst becomes less uniform and less sufficient, and as a result, there has been a problem that sufficient reaction area cannot be secured in the interface between the electrode and the ion exchange membrane.
Also, when a dispersion using an alcoholic solvent is applied onto a porous supporting material in the form of a plate, or the above ink-like dispersion is applied onto a porous supporting material, the dispersion penetrates or permeates into the inside of the supporting material. For this reason, the dispersion cannot be formed directly onto the surface of the supporting material and a complicated processing technique such as transcription and the like has been required. Further, in the method of directly applying the ink-like dispersion onto the above-mentioned membrane, a complicated producing technique of exchanging a substituent group of the membrane many times has been required. In addition, in the method of adding a fluorocarbon resin described above, there has been a problem that a catalyst fine powder is coated in excess by the fluorocarbon resin thereby to reduce reaction surface area, which impairs polarization characteristics.
On the other hand, as disclosed in aforementioned J. Electroanal. Chem. No. 197 (1986), page 195, when a carbon powder subjected to water repellency treatment with PTFE is used, a phenomenon that the catalyst particle is coated with PTFE can be suppressed. However, in this proposition, addition of a carbon powder subjected to water repellency treatment when using a polymer electrolyte and the effect thereof in respect to the addition amount have not been studied.
Furthermore, production of electrode only with a catalyst carrying carbon powder and a polymer electrolyte has presented a problem that water generated inside a fuel cell causes a so-called flooding phenomenon and operation of the cell at a high current density lowers the cell voltage and thus makes it unstable.
As a consequence, in order to permit a better performance of the cell, it has been desired to increase the reaction surface area inside the electrode by bringing the polymer electrolyte and the catalyst into contact with each other sufficiently and uniformly.
In addition, it has also been desired to provide a polymer electrolyte fuel cell that exhibits high performance even when operated at a high current density by forming a hydrogen ion channel and a gas channel without excessively coating the catalyst by addition of a fluorocarbon resin thereby improving gas permeability of the electrode.
The polymer electrolyte used for existing polymer electrolyte fuel cells exhibits the ion conductivity required when it is moist enough with water. On the other hand, the electrode reaction as a cell is a water-generation reaction in the three phase zone of the catalyst, the polymer electrolyte and the reaction gas, and if water vapor in the supplied gas or water generated by the electrode reaction is not discharged promptly and remains inside the electrode or the gas diffusion layer, gas diffusion is suppressed, thereby impairing the characteristics of the cell.
In such a viewpoint, in the electrode for use in polymer electrolyte fuel cells, measures are taken to facilitate moisturization of the polymer electrolyte and discharge of water. For example, as described above, generally employed electrode is one formed with a noble metal carrying carbon powder serving as the catalytic reaction layer on a porous electroconductive electrode supporting material serving as the gas diffusion layer. As the porous electroconductive supporting material, a carbon paper or a carbon cloth made of carbon fiber is used. In general, the porous electroconductive supporting material is primarily subjected to water repellency treatment using a dispersion of PTFE material or the like to facilitate prompt discharge of water generated by the electrode reaction, maintaining the polymer electrolyte membrane and the polymer electrolyte in the electrodes in suitably moist condition. As an alternative for this is employed a method for discharging excessively generated water in the catalytic reaction layer by mixing a carbon powder subjected to water repellency treatment into the catalytic reaction layer.
However, the above-mentioned technique has presented a problem that, although discharge of water in the gas diffusion layer is facilitated, discharge of water in the catalytic reaction layer and gas diffusion to the catalytic reaction layer are deteriorated, and particularly the characteristics of the cell are impaired when the air utilization rate is high or when discharged at a large current.
Further, when a carbon subjected to water repellency treatment with PTFE dispersion particles of submicron order is introduced in the catalytic reaction layer, a large amount of the polymer electrolyte in the catalyst reaction layer is adsorbed onto the carbon powder subjected to water repellency treatment, as previously described, and there has been a problem that the degree of contact between the polymer electrolyte and the catalyst fine particle is insufficient and not uniform, or the catalyst particle is coated with PTFE, making it impossible to secure sufficient three phase zone. Moreover, there has been another problem that, if the catalyst carrying carbon particle is water repellent, the condition of the polymer electrolyte in the polymer electrolyte membrane and the catalytic reaction layer shifts from moist condition to dry condition, thereby deteriorating the characteristics of the cell.
As a consequence, an electrode with high performance designed such that water does not stay in the catalytic reaction layers and also the polymer electrolyte is maintained in a suitably moist condition are demanded.
Thus, an object of the present invention is to solve the above-described problems by controlling the constitution of the electrodes in a fuel cell thereby to improve the efficiency of the electrode reaction in a polymer electrolyte fuel cell.
The present invention relates to a polymer electrolyte fuel cell comprising a polymer electrolyte membrane and a pair of electrodes having each a catalytic reaction layer and a gas diffusion layer, the above polymer electrolyte membrane being disposed between the pair of electrodes, wherein a part of a carrier, which carries a catalyst particle, in the above catalytic reaction layer penetrates in the inside of the above polymer electrolyte membrane.
It is effective that the above-mentioned carrier is a needle-shaped carbon fiber.
Further, the present invention relates to a polymer electrolyte fuel cell comprising a polymer electrolyte membrane, a catalytic reaction layer, and a pair of electrodes having each a catalytic reaction layer and a gas diffusion layer, the above polymer electrolyte membrane being sandwiched by the pair of electrodes, wherein the fuel cell further comprises a layer comprising an electroconductive fine particle between the above catalytic reaction layer and gas diffusion layer.
In this case, it is effective that a part of the layer comprising an electroconductive fine particle penetrates into the gas diffusion layer.
Also, it is effective that an average primary particle diameter of the above electroconductive fine particle is 10 to 100 nm.
Further, it is effective that the materials of the electroconductive fine particles, which constitute the layers comprising a conductive fine powder, are different on both sides of the polymer electrolyte membrane.
Still further, it is effective that the above electroconductive fine particle is selected from the group consisting of an electroconductive fine particle made of carbon, an electroconductive fine particle made of metal, an electroconductive fine particle made of a carbon-polymer composite and an electroconductive fine particle made of a metal-polymer composite.
It is effective that the above carbon-polymer composite is a carbon powder with PTFE adhered thereon.
It is effective that the PTFE content of the layer comprising an electroconductive fine particle falls in the range of 5 to 75% by weight.
Moreover, the present invention relates to a polymer electrolyte fuel cell comprising a polymer electrolyte membrane and a pair of electrodes having each a catalytic reaction layer and a gas diffusion layer, the above polymer electrolyte membrane being sandwiched by the pair of electrodes, wherein at least either surface of a catalyst particle or a carrier, which carries the catalyst particle, in the catalytic reaction layer has a hydrogen ion diffusion layer.
It is effective that the above hydrogen ion diffusion layer is formed by chemically bonding a silane compound to at least either surface of the catalyst particle or the carrier, which carries the catalyst particle.
It is effective that the above hydrogen ion diffusion layer comprises an organic compound having a basic functional group and a hydrogen ion conductive solid electrolyte, and that the above organic compound modifies at least either surface of the catalyst particle or the carrier, which carries the catalyst particle.
In this case, it is effective that the above basic functional group contains a nitrogen atom having a lone pair.
Also, it is effective that the above organic compound having a basic functional group is a silane compound.
Further, it is effective that the above silane compound has a functional group capable of dissociating a hydrogen ion.
Still further, it is effective that the above silane compound has at least one of a hydrocarbon chain and a fluorocarbon chain.
Moreover, it is effective that the above silane compound is chemically bonded to at least either surface of the catalyst particle or the carrier, which carries the catalyst, via at least one functional group selected from the group consisting of phenol hydroxide group, carboxyl group, lactone group, carbonyl group, quinone group and anhydride carboxylic acid group.
In addition, the present invention relates to a polymer electrolyte fuel cell comprising a polymer electrolyte membrane and a pair of electrodes having each a catalytic reaction layer and a gas diffusion layer, the above polymer electrolyte membrane being sandwiched by the pair of electrodes, wherein the above catalytic reaction layer contains at least a catalyst body comprising a hydrophilic carbon material with a catalyst particle carried thereon and a water repellent carbon material.
In this case, it is effective that a hydrophilic layer is chemically bonded to at least a part of the surface of the catalyst particle.
Further, it is effective that the above catalyst body is selectively disposed on the polymer electrolyte membrane side and the water repellent carbon material on the gas diffusion layer side, respectively in the catalytic reaction layer.
Still further, it is effective that the above water repellent carbon material has a monomolecular layer formed by chemically bonding a silane coupling agent having a hydrophobic moiety to at least a part of the carbon material surface.
Moreover, it is effective that the hydrophilic carbon material has a layer formed by chemically bonding a silane coupling agent having a hydrophilic moiety to at least a part of the carbon material surface.
In addition, it is effective that the above silane coupling agent is chemically bonded to the above carbon material surface via at least one functional group selected from the group consisting of phenol hydroxide group, carboxyl group, lactone group, carbonyl group, quinone group and anhydride carboxylic acid group.